A commutator motor used in household electric appliances such as vacuum cleaner increases in spark discharge (hereinafter called spark) generated between the brush and commutator segments during operation of the motor as the motor becomes reduced in size and weight, enhanced in performance, and increased in input, and consequently, there arises a problem of being unable to ensure the brush life.
In order to solve such a problem, it is necessary to lower the spark voltage, and to take measure such as setting the specific resistance of the brush higher and reducing the number of windings of the armature, thereby ensuring the brush life. However, it is not best to employ such measure because it results in lowering of the motor efficiency.
On the other hand, in a low voltage (3˜20V) type commutator motor used in information equipment, generally in order to ensure the brush life, there is externally provided with a disk-like spark absorbing element having varistor characteristic between the commutator segment and winding.
A varistor generally used as a spark absorbing element has such characteristic that it rapidly decreases in resistance value when the voltage exceeds a certain level and starts current application, and generally, the voltage in 1 mA current application is called varistor voltage.
A varistor generally employed for a commutator motor used in such information equipment as mentioned above is based on strontium titanate, and the varistor voltage ranges from about 3 V/mA to 20 V/mA.
When the spark absorbing element is electrically connected between winding and commutator segment, an effect of absorbing electromotive forces generated in commutation switching operation can be obtained, thereby suppressing spark energy in spark generation and making it possible to prolong the brush life. As an example of prolonging the brush life by using such a spark absorbing element, for example, Patent document 1 and Patent document 2 are disclosed. In Patent document 1, an example of using a spherical spark absorbing element is disclosed, and in Patent document 2, an example of using a pin-shaped spark absorbing element is disclosed. The latter example is described in the following with reference to the drawings. FIG. 9 is a partially sectional view of a spark absorbing element inserted portion in a conventional mold commutator. FIG. 10 is an explanatory diagram of a passage of current flowing to the spark absorbing element.
The conventional mold commutator comprises a plurality of commutator segments 80 made of copper or copper alloy and circumferentially arranged, and pin-shaped spark absorbing elements 82, and resin mold portion 85 molded them integrally. Under-cut 81 is disposed between the commutator segments to separate them from each other, thereby completing the mold commutator.
Pin-shaped spark absorbing element 82 has a function of absorbing spark voltage between commutator segments 80 and also a function of fixing and reinforcing commutator segments 80 against the centrifugal forces applied to commutator segments 80 during high-speed operation. The spark voltage in operation of the commutator motor is absorbed due to spark absorbing current 91 and spark absorbing current 92 shown by the arrows in FIG. 10.
However, there is a problem that the spark absorbing current does not uniformly flow in spark absorbing element 82. That is, spark absorbing current 91 (shown by solid line arrow) becomes far greater in value than spark absorbing current 92 (shown by broken line arrow), and consequently, the current is concentrated at contact portion 95 between spark absorbing element 82 and commutator segment 80. As a result, the varistor characteristic is greatly lowered, and the spark voltage absorbing performance is not sufficiently displayed, resulting in worsening of the brush life. Also, when used in a high-voltage motor, local self-heating around contact portion 95 is enhanced and there is a possibility of deterioration of the element.    Patent document 1 Unexamined Japanese Patent Publication H8-237913.    Patent document 2 Unexamined Japanese Patent Publication H10-257739.